In a typical memory device, data is stored at each address. The address is specified to read data stored in the specified address. In an associative memory device, data similar to input data is searched and read.
A typical associative memory is a content addressable memory (CAM) configured to search a reference data base for data completely matching with search data (refer to Patent Literatures 1 to 3 and Non Patent Literatures 1 to 3, for example). The CAM, which operates completely in parallel, includes a memory cell and a comparison circuit at each memory cell. The comparison circuit compares input data with reference data in the memory cell. The CAM searches fast for reference data matching with a specified destination IP address or MAC address. However, the CAM is useful for searching for an IP address at a network router, but is not suitable for searching for similar data.
One of technologies of searching for similar data is an associative memory having a nearest neighbor search (NNS) function. The associative memory is achieved in a digital approach (refer to Patent Literatures 4 and 5 and Non Patent Literatures 4 to 7, for example) or an analog approach (refer to Non Patent Literature 8, for example). In each approach, a region in which reference data is stored is provided by a volatile SRAM.
The group of the inventors have developed the technology of a spin transfer torque-magnetoresistance random access memory (STT-MRAM) as a nonvolatile memory (refer to Non Patent Literatures 9 and 10, for example).